1. Field of the Invention
This invention relates to an endoscope provided with an objective lens and a solid-state image pickup device in its distal end portion.
2. Description of the Related Art
In recent years, wide use has been made of endoscopes which have elongated inserting portions to be inserted into body cavities to make a diagnosis or test of internal organs. Also in the industrial or technical field, endoscopes are used to observe and examine the interior of a boiler, machine, chemical plant, or the like.
Furthermore, use has been made of various electronic endoscopes which have solid-state image pickup devices, such as CCDs. The solid-state image pickup device is arranged at the distal end of the inserting portion of an endoscope to be inserted into an examined part, for example, a body cavity. A means for mounting the solid-state image pickup device and an objective lens system for imaging an object on the solid-state image pickup device (hereinafter called "imaging system") is disclosed in, for example, Japanese Laid-Open Patent Application, Publication No. 62-31820. However, as the application fields of endoscopes have been diversified, it is desired to compact the imaging system to reduce the diameter of the distal end portion.
Thus, an imaging system as shown in FIG. 10 has been proposed. A concave objective lens 78a, convex objective lenses 78b and 78c, and a solid-state image pickup device 82 are arranged and supported in a substantially cylindrical supporting frame 77. The rear surface of the objective lens 78c is contacted by cementing or the like to a mask 81 for preventing the diffused reflection from the solid-state image pickup device 82, and an epoxy resin layer of the solid-state image pickup device 82.
In the above imaging system, however, since the objective lens 78c is contacted to the solid-state image pickup device 82, it is necessary to make the diameter of the objective lenses 78a to 78c large enough to utilize the imaging surface of the solid-state image pickup device 82. Furthermore, the supporting frame 77 should be thick because it also supports the solid-state image pickup device 82. Consequently, there is a problem that the distal end portion becomes thick.
U.S. Pat. No. 4,491,865 discloses an image sensor assembly in which a lens system is supported by a lens barrel which, in turn, is arranged in the front of a cylindrical housing, a solid state image sensor is arranged in the back of the housing, and a gas tight chamber is formed between the lens system and the image sensor. However, this structure requires the cylindrical housing for supporting the lens barrel and the solid state image sensor, thus the distal end becomes thick. There is also a problem that the assembly is elongated in the direction of the optical axis since the gas tight chamber is provided.